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Universe

stars, light, exposure, extent, faint, limit and bodies

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UNIVERSE. In the article STARS will be found a description in detail of those wonderful. bodies which stud the sky. In the present article we consider these bodies as forming a connected whole, which we call the universe, and which comprehends creation in its widest extent. Many questions connected with the universe are not yet answered; but our ideas of its structure are surely though slowly advancing. The object of the present article is to set forth what can be said on the subject at the present time.

The first question to rise is the oldest. Is the collection of heavenly bodies — stars and nebulae — which we see with our telescopes, a bounded whole of any kind; or do such bodies extend through infinite space, so that those we see are distinguished from the others only by their proximity to our system? The general trend of modern science is toward the former alternative. While it is quite true that no limit can be set to the possible extent of creation, the evidence is very strong that that portion of creation which can be studied by man -forms a bounded whole having certain common char acteristics which run through its whole extent. It seems almost certain that there is a limit in every direction beyond which the stars become comparatively few and scattered, if they exist at all. Indeed, if we consider the recent testi mony of the delicate photographic plate, there seems no escape from this conclusion. As is well known, the chemical action upon such a plate of the rays of light from the stars is cumulative, so that the longer the exposure to a selected region of the sky the larger will images of the brighter stars become and the more distinct the images of the faint ones. rt.xcessively faint stars may, in fact, not appear upon the plate at all until the exposure has been greatly prolonged, and multitudes of such ob jects can in fact be photographed in this way which are far too faint to be seen in any exist ing telescope. A photograph taken by Roberts of a region in Cygnus which was particularly rich in stars was duplicated by him two years later, but with an exposure four times as long. A careful examination of the two plates showed that no additional stars had been revealed upon the second one. The same result has since been obtained in several different portions of the sky, an exposure of 10 or 12 hours showing in Some cases but very few more images than an exposure of one or two hours. It must be

remembered that in our universe stars of all varying degrees of size and brightness are doubtless inextricably intermingled, and that a single fainter star is not necessarily more dis tant than a single brighter one, though the fainter stars are as a whole unquestionably farther away. Nevertheless it seems certain that, at least in the regions above referred to, there have been secured upon the plates records of all objects large enough to exist as self luminous suns. And if the universe is thus limited in several different directions, we must conclude that it is as a whole not of infinite extent. Another simple consideration which is often adduced as a proof of this fact is that if the stars extended outward without limit, their infinite number would fill the whole sky with a blaze of light equal to that of the noon day sun. Instead of this we have a night-sky so faint that a circle of it half a degree in diameter is almost or quite invisible to the eye. But this proof is only conclusive if we suppose the space between the stars to be absolutely free from dark matter, so that light is not perceptibly absorbed during itspassage to us. That this condition is not perfectly met is certain from many considerations, but that the amount of this dark material, whether it principally con sists of meteoric material, cosmic dust or atoms or ions of repelled gasses, produces but little effect upon light transmitted from a finite distance is evident from a study of the be havior of light of different wave lengths which comes to us from distant stars. (See LIGHT VELocrrv). But the effect, small or even un detectable though it may be, prevents our draw ing any certain conclusions in regard to a uni verse of perfectly unlimited extent. Another idea of the subject may be based on the prin ciple that in a universe of stars, extending out indefinitely, there would be nearly four times as many stars of each order of magnitude as of the order next brighter. This is true in the case of the stars visible to the naked eye. But, when counts are made of the telescopic stars, it is found that although the number of each suc cessive order increases, the ratio of increase continually diminishes, thus showing that a limit must finally be reached.

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